The present invention relates to a method of intensifying the washing of a fiber suspension. In particular, the present invention relates to decreasing the lignin content of a fiber suspension by extracting the substance whereby the suspension to be treated has a higher consistency than usually and the viscosity of the fibers is kept unchanged. In other words, the strength properties of the fibers are preserved and are the same as before the washing.
In pulp production processes, chips and digesting chemicals are dosed to the reaction vessel. The chemical reacts with the wood and these digestion reactions dissolve the lignin. The products of the reaction remain partly in the chips and partly in the surrounding liquid solution.
The reaction products of the digestion, the so-called dry substance, are to be washed off from the pulp in a washing process after digestion. The pulp which was before the washing or during the washing in the form of chips has the washing or during the washing in the form of chips has been dispersed to a fiber suspension. The fraction which has not been spread into fiber suspension, separated from the pulp before, during or after the washing, is treated and returned to the process or is removed from it as the case may be. Washing liquid is introduced to the washing process of pulp in a direction opposite the flow direction of the pulp. From the beginning of the washing plant the sludge containing solid material is guided to a chemical regeneration section and to combustion.
After digestion, washing and screening, pulp is often delignified with oxygen and bleached by using different bleaching chemicals and sequences. These treatments involve reactions between the fiber material and chemicals and reaction products are produced which should be removed in the following washing phase.
The structure of the wood fiber is known to be layered. The innermost layer is hollow lumen which is surrounded by walls containing cellulose, hemicellulose and lignin. The fiber wall has different pores depending on the fiber species and type. When chemicals contact the fibers, in particular, in digestion where the fibers are in a noncompressed state, they are absorbed by penetration and diffusion even to the lumen. Thus, reactions between the wood and the chemicals take place through the whole fiber. The dissolved and soluble reaction products remain partly in the fibers. The washing phase following the reaction phase and the conditions in the washing phase in washing plants applying modern technology are such that all the soluble reaction products cannot be removed from the fibers or there is insufficient time to remove all the soluble reaction products.
It is a well known fact that by treating pulp at an increased temperature and pressure, the reaction product contained in a soluble form in the fiber can be removed. A treatment of this type provides remarkable advantages compared with conventional treatments. For example, the kappa number of the pulp to be bleached, in other words, its lignin content, is lower which saves bleaching chemicals and decreases the impact of the process on the environment. After bleaching and oxygen treatment, the same advantages are to be achieved with a treatment of the same type. However, the prior art methods have their drawbacks. Usually, lignin is removed at a consistency of less than 3%, at which consistency pulp warms up slowly and the heating requires energy many times as much as heating of pulp of, for instance, the consistency of 10%. The following example clarifies the difference. In a fiber suspension having a consistency of 3%, there is about 32 kg's liquid per one kg of fibers and in a suspension having a consistency of 10%, there is 9 kg's liquid per one kg of fibers. Thus, to heat one kg of fibers in a consistency of 3%, one needs to heat 23 kg's more liquid than in the case of heating pulp in a consistency of 10%. A more serious problem in particular in the paper manufacture is the decrease of the viscosity of the fiber suspension as illustrated in FIGS. 6 and 7. This indicates that the strength properties of the fibers decrease considerably when lignin is removed at a low consistency. For example, at the consistency of 10%, the strength properties do not change substantially.